Organometallic compounds of elements from transition group 4 are known. They are used, for example, as crosslinking catalysts in the preparation of polyurethanes, more particularly in solvent-based systems.
WO 98/41322 A1 describes various organometallic complexes of zirconium and hafnium. They are used for producing (poly)urethanes for adhesives, foams, coatings, sealants, and plastic goods which are produced, for example, by a spraygun process and by an injection molding process. The formulations of WO 98/41322 A1 are based without exception on organic solvents.
US 2004/0220326 A1 describes catalysts for urethane crosslinking in aqueous two-component polyurethane coating materials. Used for this purpose are organic metal complexes from transition groups 5 and 6.
WO 00/34355 describes Lewis acid catalysts, including organometallic compounds of various main group and transition group elements. They are used for producing curable, uretdione-containing compositions which cure at low temperatures. These compositions are more particularly powder coating materials.
US 2007/0010644 A1 describes the production of elastomers. Through the simultaneous use of two different catalysts, including an organic metal complex, relatively fast-curing elastomers are obtained which on injection exhibit fewer dripping losses, and whose elastomeric products can be demolded after a shorter time.
He et al. (Journal of Coatings Technology 2002, 74 (930), 31-36) describe a two-component polyurethane (2-K-PU) system that comprises metal carboxylates or metal β-diketonates. The rate of urethane formation by butyl isocyanate with 2-ethylhexanol is investigated, in comparison to the formation of urea with water. In this system, zirconium compounds prove to be good catalysts of urethane formation. As described, however, the zirconium diketones are hydrolyzed over time in the aqueous system, leading to deactivation of the zirconium catalyst in the aqueous medium. The coating is applied to the substrate by drawing with a wire applicator before subsequent curing. He et al. further describe the use of a zirconium diketonate in a clearcoat material based on a polyester dispersion, and in a solventborne two-component acrylate emulsion. In these cases an improved gloss)(20°) is achieved in the resulting coating.
Florio et al. (Paint and Coatings Industry 2000, 16, 80-94) describe various tin-free organic catalysts of urethane formation, including organic compounds of bismuth, aluminum, and zirconium. This series of organometallic compounds is available commercially under the “K-KAT®” designation. Florio et al. describe suitable fields for use of these bismuth, aluminum, and zirconium compounds. While the bismuth compounds (K-KAT 348, K-KAT XC-B203) appear suitable for use in electrodeposition coatings, the problem of catalyst deactivation by hydrolysis must be borne in mind as a general consideration. Deactivation of the catalysts used may occur specifically due to water and anions, more particularly phosphate ions, and may not become apparent in the coating product until after a few days. Polyether polyols, for example, may comprise phosphate ions as a contaminant from the production process. As Florio et al. describe, phosphate ions can induce the deactivation of the stated catalysts, more particularly that of zirconium catalysts.
Cathodically depositable electrodeposition coating materials are suitable for use in cathodic electrodeposition coating (cathodic electrocoat). Cathodic electrocoat is a coating process frequently employed in particular for priming, where binders which carry cationic groups, in dispersion or solution in water, are applied using direct current to electrically conducting articles. For that purpose the substrate for coating is connected as a cathode and is immersed into the cathodically depositable electrodeposition coating material. When a direct current is applied between the substrate, connected as the cathode, and an anode which is likewise situated in the electrodeposition coating material, the charged paint micelles or dispersion particles pass, within a diffusion-controlled boundary layer, to the oppositely charged electrode, where they are precipitated through pH change resulting from the electrolytic decomposition of the water. If the surface charge of the paint micelles or dispersion particles is positive, deposition occurs on the cathode, i.e., on the substrate connected as the cathode. The paint film deposited has a high solids content and, following the removal of the substrate from the dipping tank, and optional cleaning steps, is typically crosslinked by baking.
The cathodically depositable electrodeposition coating materials that are in use nowadays customarily meet exacting requirements in terms of corrosion protection, edge protection, surface quality, and other properties, such as sandability, for example. A further requirement, however, is that a cathodic dip coating should have very few craters (crater-shaped film defects with depressions which can reach down almost to the substrate, and which remain after drying). Craters are formed more particularly in the presence of a contaminant whose surface tension is lower than that of the paint film. Immediately after deposition, no craters are apparent. They only appear in conjunction with the flow phenomena during the baking operation. Defects of this kind in a cathodic electrodeposition coating may possess diameters of up to several millimeters. Often they can still be perceived on the paint surface after topcoating has taken place, and they therefore entail laborious and costly afterwork.
In spite of a very wide variety of technical measures, such as reducing the film thickness, raising the pigment content of the electrocoat material, increasing the paint viscosity, and thoroughly cleaning and degreasing the substrates, for example, it has to date not proved possible to reliably avoid these defects. A tendency toward cratering appears to be especially pronounced more particularly in cathodic electrocoat materials that contain tin catalyst. One particular source of disruption is the contamination of the cathodic electrocoat bath with phosphate ions. The substrates used for cathodic electrocoating are typically pretreated by means of a phosphating operation. The phosphated substrates are then introduced into the electrocoat bath for coating. Owing to phosphate entrainments from the pretreatment operation, therefore, the cathodic electrocoat bath may be affected by phosphate ion contamination which is virtually unavoidable by conventional means. This problem exists more particularly in the painting lines that are typically used in the industrial cathodic electrodeposition coating process.
The object, therefore, is to provide a cathodic electrodeposition coating material that exhibits a reduced tendency to form craters, with the consequence that a very largely crater-free coating can be obtained even, more particularly, in the case of the cathodic electrocoating with phosphated substrates and the attendant possibility of contamination of the cathodic electrocoat bath with phosphate ions. This allows laborious and costly afterwork to be avoided and hence can make the production of correspondingly coated substrates more time-efficient and cost-effective.